1. Field of the Invention
The present invention relates to a liquid crystal element of optical writing type, and more particularly to the liquid crystal element of optical writing type which is preferably adapted to a projecting type liquid crystal display apparatus arranged to project an image onto a light-reflective screen, a sensor for an image scanner, and a wavelength converting element for converting near infrared light into visible light, for example.
2. Description of the Related Art
The representative related art about the liquid crystal element of optical writing type known by the present inventors will be described in this item.
The liquid crystal element of optical writing type is composed to have glass substrates on both of the outer sides, transparent electrodes formed on the inner sides of the respective glass substrates, a photoconductive layer formed on one of the transparent electrodes, a dielectric mirror layer formed on the photoconductive layer, orientation films formed respectively on the dielectric mirror layer and the other transparent electrode, and a liquid crystal layer disposed between the orientation films.
The photoconductive layer is made of hydrogenated amorphous silicon (referred to as a-Si:H), which layer is formed by means of a plasma CVD method using silane gas and hydrogen gas as raw materials. The dielectric mirror is formed of multi-layered films composed of silicon or silicon oxide by means of a sputtering method. The orientation film is formed on a polyimide film by means of a spin coating technique. The liquid crystal layer is made of nematic liquid crystal.
This kind of liquid crystal element of optical writing type is allowed to have some operation modes such as a twisted nematic (TN) mode, a hybrid field effect (HFE) mode, a guest host (GH) mode, and a phase transition mode.
In operation, an alternating voltage is applied between the transparent electrodes. When a ray of light is emitted from a CRT (Cathode Ray Tube) to one of the glass substrate, the photoconductive layer serves to lower its impedance at the light-hit area (bright condition) so that the applied voltage is strong enough to drive the liquid crystal layer, and keeps its impedance at the other area where no light is hit (dark condition) so that the applied voltage is not strong enough to drive the liquid crystal layer. The contrast between the bright condition and the dark condition results in forming an image.
The foregoing related art uses the a-Si:H layer as its photoconductive layer. The a-Si:H layer has the similar magnitude of dark conductivity as the conductivity of the liquid crystal. It means that the photoconductive layer has the similar magnitude of the impedance as the liquid crystal layer so that a certain amount of voltage is applied to the liquid crystal layer in the dark condition. When a ray of light is hit onto this photoconductive layer, that is, the light-hit area of the photoconductive layer becomes the bright condition, the photoconductive layer lowers its impedance so that the voltage applied to the liquid crystal layer becomes strong enough to drive the liquid crystal at the corresponding portion. However, in this structure, since the impedance of the photoconductive layer is in the similar magnitude as that of the liquid crystal layer as stated above, the ratio of a voltage applied to the liquid crystal layer between the dark condition and the bright condition (on/off voltage ratio) is too small to obtain a high contrast image.
To overcome this shortcoming, the photoconductive layer (a-Si:H layer) may be composed to have Schottky structure or diode structure. The use of such structure leads to the reduction of the voltage applied to the liquid crystal layer in the dark condition where the impedance of the photoconductive layer is made high since it is reverse-biased. It results in enlarging an on/off voltage ratio of the liquid crystal layer between the bright condition and the dark condition. However, this structure has the drawback of applying d. c. voltage component to the liquid crystal. This application of the d. c. voltage to the liquid crystal cell often results in decomposing the liquid crystal material itself and causing attraction of ion components contained in the liquid crystal to the glass substrate, thereby causing the disorder of the molecular orientation and the degradation of the characteristic of the liquid crystal.